[Michael Hancock] “If I add metadata to EventA (notes, scene info, favorites, etc…), can I transfer and merge that with EventA on the other editor’s system, without having duplicate clips added to their event? “
Yes this can be done by exporting the event XML then importing it on the destination machine. Select the event, then do File>Export XML. On the destination machine, do File>Import>XML. At import time it will give you the option to overwrite the existing metadata or create duplicate clips. There is no intra-clip overlay-type metadata merge. It either uses the source machine’s metadata or it makes duplicate clips, preserving both original and new metadata. See https://support.apple.com/kb/PH12654?locale=en_US
You should definitely make a backup copy of the library before doing this.
Even if it seems to work, give the library a close look and verify several clips work correctly, the metadata works as expected and the clips are in the correct event. If you have other events, check that those were not changed. You don’t want to load the XML, believe it’s OK, then commit lots of work only to find out something went wrong that you didn’t spot.